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    <script>
        // 7、求1 + 2！ + 3！ + 4！....+20!的值
        // - 2！ = 2 * 1
        // - 3！ = 3 * 2 * 1

        var res = 1
        for (var i = 1 ; i <= 20 ;i++) {
            res *= i
        }
        console.log(res)

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